This relates generally to integrated circuits, and more particularly, to programmable integrated circuits with decoupling capacitors.
Programmable integrated circuits are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. In a typical scenario, a logic designer uses computer-aided design (CAD) tools to design a custom logic circuit. When the design process is complete, the tools generate configuration data. The configuration data is loaded into memory elements (sometimes referred to as configuration memory cells) to configure the device to perform the functions of the custom logic.
During normal operation of a programmable device, loaded configuration memory cells produce static output signals that are applied to the gates of transistors (i.e., pass transistors). The configuration memory cell output signals turn some pass transistors on and turn other pass transistors off. This selective activation of pass transistors on the programmable device customizes the operation of the device so that the device performs its intended function.
Programmable integrated circuits typically include decoupling capacitors. Decoupling capacitors are used to help provide more stable power supply voltages by shunting high frequency noise on direct current (DC) power supply lines to ground, thereby preventing the noise from reaching powered circuit components. In a scenario in which a power supply is required to switch between various modes of operation, an adequate decoupling capacitance can act as an energy reserve that lessens the magnitude of undesired dips in power supply voltage during mode switching events.
To ensure that a wide range of logic functions can be implemented, programmable integrated circuits are equipped with large amounts of configurable resources sufficient to implement any desired custom logic functions for a vast majority of users. As a result, programmable integrated circuits often include unused resources such as unused routing paths, memory cells, and other switching circuits. Conventionally, these unused routing paths are driven to a positive power supply voltage level. Each unused routing path that is driven as such will exhibit a parasitic capacitance to ground. Parasitic capacitance generated in this way provides additional decoupling capacitance for the programmable integrated circuit.
Advances in integrated circuit design require power supplies to supply stable power for integrated circuits operating at high data rates and clock speeds. This requires increasing amounts of decoupling capacitance per integrated circuit area. Decoupling capacitance provided by driving the unused routing paths to only the positive power supply voltage level may not be sufficient. Additional decoupling capacitor circuitry could occupy a disproportionate amount of valuable surface area on the programmable device.